1. Technical Field
The present invention relates to a communication system, and more particularly, to a method for generating a hybrid codebook and a hybrid Multi-User Multiple-Input Multiple-Output (MU-MIMO) system.
2. Related Art
An MIMO scheme is one of methods for increasing data transmission efficiency in a wireless communication system. The MIMO scheme can be classified into a Single-User MIMO (SU-MIMO) scheme and a Multi-User MIMO (MU-MIMO) scheme, depending on whether data can be transmitted to multiple users by using the same resource. The MU-MIMO scheme can transmit different data to multiple users simultaneously by using the same resource, and it has been known that the MU-MIMO scheme can obtain higher spectral efficiency than the SU-MIMO scheme because of multi-user diversity gain and spatial multiplexing gain thereof.
An MIMO system can be classified into an open-loop scheme in which a transmitter performs a communication without knowing a channel condition, and a closed-loop scheme in which a transmitter performs a communication while referring to channel information fed back from a receiver. The closed-loop scheme is widely used because it can approach a theoretical transmission capacity by applying independent modulation and coding schemes to transmit antennas according to channel states.
In the closed-loop MU-MIMO system, a receiver can use a codebook to send channel information to a transmitter. Codewords constructing the codebook represent different channel states of channels formed between the transmitter and the receiver. The receiver estimates a channel by using a pilot signal received from the transmitter, selects a codeword corresponding to the estimated channel, and informs the transmitter of the channel state by feeding a corresponding codeword index back to the transmitter. That is, in a case in which beamforming is performed at a base station by using column vectors of the codebook existing in the receiver as beamforming vectors, downlink channel quality is calculated to generate downlink channel quality indicators, and then, a position of a column vector corresponding to the best downlink channel quality indicator and the corresponding downlink channel quality indicator are fed back to the transmitter.
Accordingly, the transmitter selects receivers, which are to receive data, as many as the number of transmit antennas in order to maximize a downlink data rate, considering the fed-back downlink channel quality indicator and the index of the corresponding beamforming vector.
One of beamforming methods using a codebook is Per User Unitary Rate Control (PU2RC). The PU2RC scheme uses a codebook constructed by several appointed unitary precoding matrices and transmits data by selecting the receiver group having the best reception quality through grouping of receivers using the same precoding matrix. Since the PU2RC scheme determines a receiver and a precoding matrix in consideration of only a single-cell environment, its performance may be limited by interference between base stations in a multi-base station environment. In addition, since the precoding vector is determined in such a state that the codewords is constructing the set of beamforming vectors, that is, the codebook, have already been determined, interference between receivers may not be completely eliminated. Moreover, in order to simplify the architecture of the system and obtain superior performance, it is necessary to increase the number of receivers which provide feedback information to transmitters.
Another example of the beamforming method using the codebook is Zero-Force Beam Forming (ZFBF). In the ZFBF scheme, a receiver estimates a channel from a pilot signal received from a transmitter, quantizes the estimated channel, and feeds an index of channel state information, based on the quantization result, back to the transmitter. In the ZFBF scheme, a precoding matrix is not fixed and interference between receivers can be made zero. However, since accurate channel state information is needed in order to eliminate the interference, the number of feedback bits is increased. Since beamforming information is calculated based on feedback information in real time, system complexity is increased.
As such, the PU2RC scheme and the ZFBF scheme have opposite advantages and disadvantages. In particular, in order to obtain superior system performance, the ZFBF scheme needs more feedback bits, whereas the PU2RC scheme needs more receivers. Since the two schemes have the opposite features, it cannot be guaranteed which of the two schemes is better.